To achieve the low noise and wide bandwidth required for millimeter wavelength astronomy applications, superconductor-insulator-superconductor (SIS) mixer based receiver systems have typically been ...used. This paper investigates the performance of high electron mobility transistor (HEMT) based low noise amplifiers (LNAs) as an alternative approach for systems operating in the 125 — 211 GHz frequency range. A four-stage, common-source, unconditionally stable monolithic microwave integrated circuit (MMIC) design is presented using the state-of-the-art 35 nm indium phosphide HEMT process from Northrop Grumman Corporation. The simulated MMIC achieves noise temperature (T
e
) lower than 58 K across the operational bandwidth, with average T
e
of 38.8 K (corresponding to less than 5 times the quantum limit (hf/k) at 170 GHz) and forward transmission of 20.5 ± 0.85 dB. Input and output reflection coefficients are better than -6 and -12 dB, respectively, across the desired bandwidth. To the authors knowledge, no LNA currently operates across the entirety of this frequency range. Successful fabrication and implementation of this LNA would challenge the dominance SIS mixers have on sub-THz receivers.
ABSTRACT We report here a study of gas, dust, and star formation rates (SFRs) in the molecular cloud complexes (MCCs) surrounding the giant H ii region RCW 106 using 12CO and 13CO(1-0) data from the ...Three-mm Ultimate Mopra Milky way Survey and archival data. We separate the emission in the Galactic Plane around l = 330°-335° and b = −1°-1° into two main MCCs: the RCW 106 (VLSR = −48 km s−1) complex and the MCC331-90 (VLSR = −90 km s−1) complex. While RCW 106 (M ) is located in the Scutum-Centaurus arm at a distance of 3.6 kpc, MCC331-90 (M ) is in the Norma arm at a distance of 5 kpc. Their molecular gas mass surface densities are ∼220 and ∼130 pc−2, respectively. For the RCW 106 complex, using the 21 cm continuum fluxes and dense clumps counting, we obtain immediate past (∼−0.2 Myr) and immediate future (∼+0.2 Myr) SFRs of and . This results in an immediate past SFR density of and an immediate future SFR density of . As both SFRs are higher than the ministarburst threshold, they must be undergoing a ministarburst event although burst peak has already passed. This is one of the most active star forming complexes in the southern sky that is ideal for further investigations of massive star formation and potentially shedding light on the physics of high-redshift starbursts.
This study investigate the effectiveness of using Deep Learning (DL) for the classification of planetary nebulae (PNe). It focusses on distinguishing PNe from other types of objects, as well as their ...morphological classification. We adopted the deep transfer learning approach using three ImageNet pre-trained algorithms. This study was conducted using images from the Hong Kong/Australian Astronomical Observatory/Strasbourg Observatory H-alpha Planetary Nebula research platform database (HASH DB) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). We found that the algorithm has high success in distinguishing True PNe from other types of objects even without any parameter tuning. The Matthews correlation coefficient is 0.9. Our analysis shows that DenseNet201 is the most effective DL algorithm. For the morphological classification, we found for three classes, Bipolar, Elliptical and Round, half of objects are correctly classified. Further improvement may require more data and/or training. We discuss the trade-offs and potential avenues for future work and conclude that deep transfer learning can be utilized to classify wide-field astronomical images.
The formation of massive stars passes through a so-called hot molecular core phase, where the temperature of molecular gas and dust rises to above 100 K within a size scale of approximately 0.1 pc. ...The hot molecular cores are rich in chemical compounds found in the gas phase, which are a great probe of ongoing star formation. To study the impact of the initial effects of metallicity (i.e., the abundance of elements heavier than helium) on star formation and the formation of different molecular species, we searched for hot molecular cores in the sub-solar metallicity environment of the Large Magellanic Cloud (LMC). We conducted Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations of 20 fields centered on young stellar objects (YSOs) distributed over the LMC in order to search for hot molecular cores in this galaxy. We detected a total of 65 compact 1.2 mm continuum cores in the 20 ALMA fields and analyzed their spectra with XCLASS software. The main temperature tracers are CH3OH and SO2 with more than two transitions detected in the observed frequency ranges. Other molecular lines with high detection rates in our sample are CS SO H^ CO^+ H^ CN HC^ N and SiO . More complex molecules, such as HNCO HDCO HC3N CH3CN and NH2CHO and multiple transitions of SO and SO2 isotopologues showed tentative or definite detection toward a small subset of the cores. According to the chemical richness of the cores and high temperatures from the XCLASS fitting, we report the detection of four hot cores and one hot core candidate. With one new hot core detection in this study, the number of detected hot cores in the LMC increases to seven. Six out of seven hot cores detected in the LMC to date are located in the stellar bar region of this galaxy. These six hot cores show emission from complex organic molecules (COMs), such as CH3OH CH3CN CH3OCHO and CH3OCH3 . The only known hot core in the LMC with no detection of COMs is located outside the bar region. The metallicity in the LMC presents a shallow gradient increasing from outer regions toward the bar. Various studies emphasize the interaction between the LMC and the Small Magellanic Cloud, which resulted in the mixing and inhomogeneity of the interstellar medium of the two galaxies. These interactions triggered a new generation of star formation in the LMC. We suggest that the formation of hot molecular cores containing COMs ensues from the new generation of stars forming in the more metal-rich environment of the LMC bar.
We present narrow and broad K-band observations of the Class 0/I source IRAS 18148-0440 that span 17 years. The infrared nebula associated with this protostar in the L483 dark cloud is both ...morphologically and photometrically variable on a timescale of only a few months. This nebula appears to be an infrared analog to other well known optically visible variable nebulae associated with young stars, such as Hubble's Variable Nebula. Along with Cepheus A, this is one of the first large variable nebulae to be found that is only visible in the infrared. The variability of this nebula is most likely due to changing illumination of the cloud rather than any motion of the structure in the nebula. Both morphological and photometric changes are observed on a timescale only a few times longer than the light crossing time of the nebula, suggesting very rapid intrinsic changes in the illumination of the nebula. Our narrowband observations also found that H2 knots are found nearly twice as far to the east of the source as to its west, and that H2 emission extends farther east of the source than the previously known CO outflow.
W51 Main/South is one of the brightest and richest high-mass star-forming regions (SFR) in the complex W51. It is known to host many ultra-compact HII (UCHII) regions thought to be the site of ...massive young stellar objects. Maser emission from various species is also found in the region. We have performed MERLIN astrometric observations of excited-OH maser emission at 6.035 GHz and Class II methanol maser emission at 6.668 GHz towards W51 to investigate the relationship between the maser emission and the compact continuum sources in this SFR complex. Here we present the astrometric distributions of both 6.668-GHz methanol and 6.035-GHz excited-OH maser emission in the W51 Main/South region. The location of maser emission in the two lines is compared with that of previously published OH groundstate emission. The interesting coherent velocity and spatial structure observed in the methanol maser distribution as well as the relationship of the masers to infall or outflow in the region are discussed. It appears that the masers are excited by multiple objects potentially at different stages of evolution.
High-mass star formation Klaassen, Pamela D; Mottram, Joseph C; Longmore, Steven N ...
Astronomy & geophysics : the journal of the Royal Astronomical Society,
06/2013, Volume:
54, Issue:
3
Journal Article
Peer reviewed
Pamela D Klaassen and Joseph Mottram report on a workshop at the University of Leiden which discussed the formation of high-mass stars from large to small scales in the era of Herschel and ALMA. ...PUBLICATIONABSTRACT
We present one of the first Shanghai Tian Ma Radio Telescope (TMRT) K Band observations towards a sample of 26 infrared dark clouds (IRDCs). We observed the (1,1), (2,2), (3,3), and (4,4) transitions ...of NH.sub.3 together with CCS (2.sub.1->1.sub.0) and HC.sub.3N J = 2-1, simultaneously. The survey dramatically increases the existing CCS-detected IRDC sample from 8 to 23, enabling a better statistical study of the ratios of carbon-chain molecules (CCM) to N-bearing molecules in IRDCs. With the newly developed hyperfine group ratio (HFGR) method of fitting NH.sub.3 inversion lines, we found the gas temperature to be between 10 and 18 K. The column density ratios of CCS to NH.sub.3 for most of the IRDCs are less than 10.sub.-2, distinguishing IRDCs from low-mass star-forming regions. We carried out chemical evolution simulations based on a three-phase chemical model NAUTILUS. Our measurements of the column density ratios between CCM and NH.sub.3 are consistent with chemical evolutionary ages of ?? 10.sup.5 yr in the models. Comparisons of the data and chemical models suggest that CCS, HC.sub.3N, and NH.sub.3 are sensitive to the chemical evolutionary stages of the sources. star formation, infrared dark clouds, chemical evolution PACS number(s): 98.35.Ac, 98.38.Dq, 98.35.Bd
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